An illumination device as described in the previous paragraph is known. For example, U.S. Pat. No. 6,874,924 discloses a device with an elongated and substantially cylindrical waveguide, which is at a distance r from an imaginary central axis (not shown). The waveguide has a light-emitting outer surface and an interior semi-reflective portion spaced at a predetermined distance from the light-emitting outer surface. A channel is defined through a portion of the waveguide along its length. In the channel, light sources formed as LEDs are positioned. Light generated by these LEDs enters the waveguide through one or both of the flat side walls defined by the channel. The entered light is scattered in the waveguide and emitted via its curved outer surface. In the disclosed illumination device, the light is emitted substantially homogeneously and with uniform light intensity from the outer surface.
The known illumination device shows several disadvantages. A first disadvantage concerns the problem of back-reflection of light into the LEDs. Due to the design of the waveguide, a large part of the light generated by the LEDs and entered into the waveguide via the flat entrance surface(s) of the channel is reflected back into the LEDs. During operation of the illumination device, such back-reflections cause considerable optical loss. A second disadvantage concerns device costs. The devices according to the known design need additional measures causing light scattering in order to produce sufficient output and uniform light emission. Due to these additional measures, the prior art illumination devices are rather expensive. Finally, with illumination devices according to the disclosed construction, desired beam shaping can hardly be realized or not at all.